The invention is in the field of bearings. More particularly, the invention is a self-lubricating thrust bearing having a plurality of axially-aligned parts. The bearing is preferably in cartridge form and includes an internal circular spring. When the bearing loading is low, the spring expands and urges the bearing""s mating surfaces together, while also causing a positive contact between the bearing and the structure to which it is secured. Once the bearing loading exceeds a predetermined amount, the spring""s location and extent of compression prevent it from interfering with the rotational movement of any portion of the bearing.
Bearings are often employed in structures that are subject to variable loading. In some situations, the loading of the bearing will range from zero to an extreme amount. Throughout the entire load range, the bearing must maintain its structural integrity and preferably also maintain positive contact with adjacent structure. In addition, the amount of load acting on the bearing, in combination with the bearing""s design, can affect the performance characteristics of the structure in which the bearing is installed.
One example of a structure having a bearing subject to extreme variations in loading is found in the nose landing gear of an airplane. The nose landing gear, also herein referred to as the nose gear, includes the plane""s front wheel(s) and a strut assembly that attaches the wheel(s) to the plane. To facilitate turning of the wheel(s), the strut assembly includes a bearing having relatively rotatable portions. When the plane is on the ground, a significant loading of the bearing is normally present due to the portion of the plane""s weight carried by the nose gear. During and immediately after take off, the load applied to the bearing due to the wheel""s contact with the ground ranges from significant to essentially zero. During a landing, the bearing loading may also vary significantly. To enable precise wheel movements, the bearing must allow smooth rotation without a significant amount of play. The bearing must also be durable and capable of functioning in an environment subject to water, dust and/or other materials often found on the runways of any airport.
Vibration is a problem sometimes associated with structures subject to variable/cyclical loading. In an airplane, the nose gear may experience unwanted vibration during the final stages of a take-off. This occurs as the load due to the wheel""s contact with the runway becomes reduced to essentially zero immediately after lift-off. Any unbalance in the nose gear""s wheel(s), as well as forces applied by the air stream, may cause vibration in the structural elements of the nose gear that can be transmitted to the airframe and the plane""s occupants. A similar intermittent/reduced loading of the nose gear can occur during landings. Any vibration in the nose gear may be transmitted into the associated tire(s), and thereby cause an increased rate of wear while the tire is still in contact with the runway. The vibration may also be transmitted into the plane""s steering and suspension systems.
Another problem sometimes associated with structures subject to variable loading is that when the loading becomes reduced to essentially zero, any bearing in the structure may lose positive contact with the surfaces to which it is normally engaged. The lack of positive contact may produce openings into which contaminants may enter. These contaminants can adversely affect the bearing and any adjacent structure.
The invention is a self-lubricating thrust bearing having a plurality of axially-aligned parts. The bearing is preferably in cartridge form and includes first and second bearing surfaces separated by a disk of lubricating material. In the preferred embodiment, the first bearing surface forms a center portion of a first thrust member that extends outwardly of, and wraps about, the periphery of a second thrust member that includes the second bearing surface.
Unlike the bearings of the prior art, one of the bearing surfaces is incorporated into a multi-part thrust assembly that features a circular spring sandwiched between the second thrust member and a third thrust member. The spring functions to apply a load to the bearing surfaces irrespective of any exterior loading on the bearing. When the external loading is very low, the spring expands the bearing structure in a direction parallel to the bearing""s axis, thereby assuring positive contact with, and preferably applying a load to, any structure mated to the first and third thrust members of the bearing. As the bearing becomes loaded by external force(s), the spring becomes compressed to an extent whereby it no longer separates the thrust members of the thrust assembly. Once they contact each other, friction between the thrust assembly""s two thrust members is greater than between the first and second bearing surfaces, thereby causing the thrust assembly""s thrust members to essentially lock together in a substantially slippage-free engagement. The thrust assembly then acts as a single member and allows the bearing to operate in a conventional manner.
In the preferred embodiment, the spring is in the form of a circular wave washer. The washer is preferably at least partially received within an annular groove in one of the thrust members that sandwich the spring. By selecting a suitable spring rate for the spring, the spring will act to dampen unwanted vibrations created by connected structure, such as an unbalanced wheel of an aircraft""s nose gear.
The ability for low load conditions to create vibration in the bearing and/or the structure connected to the bearing is mitigated by the force applied by the bearing""s internal spring to the bearing""s components, and any attached structure. When the bearing is employed in an airplane""s nose gear, the bearing functions to minimize or dampen the vibrations that may occur during takeoffs and landings. In this manner, the life of the plane""s nose gear""s tire(s) may be increased, as well as possibly reducing vibration in the plane""s steering and suspension components.
As an option in the preferred embodiment, one or more o-rings may be employed within the bearing to prevent entry of foreign matter into the unit. As another option, the assembly may include exterior-viewable indicia that facilitates a user""s ability to determine the amount of preload acting on the bearing and/or bearing wear.